In gas turbine engines, some of the aerofoil blades and in particular the turbine blades are conventionally surrounded by a sealing structure, which may comprise an annular seal or a seal segment ring made up of a plurality of arc shaped seal segments. Because the turbine blades expand and contract as their temperatures vary in use and centrifugal loads are imposed upon them, it is normal to provide a small gap between the turbine blade tips and the seal, to allow for this fluctuation.
It is known to provide an abradable seal for sealing between the turbine blade tips and the sealing structure. This enables the tips of the turbine blades to wear away the seal to an optimum size and shape without causing damage to the turbine blade tips. Such abradable seals may consist of an open cell foil honeycomb which is brazed in place and subsequently filled with a suitable abradable material, such as a metallic powder. As stated in EP A 1146204 it is also known to directly machine, perhaps by electro discharge machining (EDM), a sealing segment made from an oxidation resistant alloy to form a honeycomb structure that is also filled with a suitable abradable material, such as a metallic powder. In both cases the honeycomb acts as a support for the abradable material. The supporting honeycomb is subsequently partially worn away by the rotating turbine blades, thus forming a seal.
Certain problems are associated with the above seals. The seals may suffer from progressive oxidation attack if the foil material has inadequate oxidation resistance. In addition, problems may be experienced with the brazed joints, and the seals may be difficult to cool.
Additional problems arise when the honeycomb structure has worn and needs to be refurbished. Sealing components produced by vacuum brazing thin foil honeycomb structures to a sealing segment are refurbished by machining away remnants of the worn honeycomb and re attaching a new honeycomb via vacuum brazing. While the brazing quality may be adequate for low temperature applications, at elevated temperatures the brazing will lose its integrity and fail, thereby limiting this technique to low temperature applications.
Where abradable portions have been machined from a solid sealing segment it is required to replace the seal segment in its entirety, thereby adding to the overall cost of the refurbishment process.
EP A 1452696 proposes a different approach in which a radially inner surface region of a sealing element includes a seal structure formed as a plurality of inwardly projecting walls formed by powder fed laser weld deposition. This approach allows cell structures to be produced which are not achievable by other machining methods, such as EDM. Additionally laser welding avoids a need for expensive tooling, permits a larger range of materials to be used for forming the cell structure, and can be used to repair worn seal structures.